1. Field of the Invention
The present invention relates to a positive electrode active material employed in nonaqueous electrolyte secondary battery such as a lithium ion secondary battery suitably used for cellular phones, personal computers, and electric vehicle, for example.
2. Description of the Related Art
A nonaqueous electrolyte secondary battery is featured to have a high operating voltage and a high energy density as compared with a conventional nickel-cadmium secondary battery and the like, and has been widely used as a power source or the like for mobile electronic appliances such as cellular phones, laptop personal computers, and digital cameras. A positive electrode active material of the nonaqueous electrolyte secondary battery includes lithium-transition metal composite oxides represented by LiCoO2, LiNiO2, and LiMn2O4. Of those, LiCoO2 has been conventionally used for the mobile electronic appliances, providing sufficient battery characteristics.
However, an environment in which the mobile electronic appliances are used has now become more harsh resulting from high-functionalization such as provisions of various functions, uses in high temperatures or low temperatures, or the like. Further, application of the nonaqueous electrolyte secondary battery to power sources such as batteries for electric vehicles is expected. However, sufficient battery characteristics cannot be obtained with a conventional nonaqueous electrolyte secondary battery employing LiCoO2, and further improvement is required.
JP 4-319260A (the term “JP XX-XXXXXX A” as used herein means an “unexamined published Japanese patent application”) describes a positive electrode consisting of Li1-xCoO2 (0≦x<1) with zirconium (Zr) added or with a part of its cobalt substituted with other transition metals. JP 4-319260 A describes that covering surface of LiCoO2 particle with zirconium oxide (ZrO2) or with a composite oxide of lithium and zirconium (Li2ZrO3) provides a stable positive electrode. As a result, a positive electrode active material exhibiting excellent cycle characteristics and storage characteristics can be obtained without causing a decomposition reaction of an electrolytic solution or crystal destruction even at high potentials.
However, this positive electrode active material could not meet the high rate characteristics, low-temperature characteristics, and thermal stability required for a recent nonaqueous electrolyte secondary battery. Further, there was a room for improvement in the cycle characteristics as well.
Further, JP 6-168722 A describes a use of LiMgxCo1-xO2-y (0<x<1, 0<y<0.5, and x=2y) as a positive electrode active material. JP 6-168722 A further describes that substituting a part of cobalt with magnesium can solve a problem of an increase in internal resistance of the battery during high discharge, which reduces discharge capacity.
However, sufficient cycle characteristics and low-temperature characteristics could not be obtained with this positive electrode active material.
Further, JP 2002-151078 A describes a positive electrode active material for a nonaqueous electrolyte secondary battery featured to have a part of particle surface of lithium cobaltate particle powder covered with titanium oxide and/or lithium titanate and to be covered with 2.0 to 4.0 mol % (as Ti) of the titanium oxide and/or the lithium titanate with respect to cobalt in the lithium cobaltate particle powder. JP 2002-151078 A further describes that covering a part of the lithium cobaltate particle surface with titanium oxide and/or lithium titanate provides an excellent charge-discharge cycle characteristics in high-temperatures while retaining initial discharge capacity for a secondary battery.
However, sufficient cycle characteristics and initial charge-discharge capacity could not be obtained with the positive electrode active material. Further, the positive electrode active material could not satisfy the high rate characteristics and average potential required for a recent nonaqueous electrolyte secondary battery.